New research from UCLA and the University of New Mexico suggests that members of "the gentler sex" may have evolved to cheat on their mates during the most fertile part of their cycle — but only when those mates are less sexually attractive than other men.

"Women know they have attractions that come and go, but they probably don't realize that these urges are tied to their cycle — as well as our evolutionary past," said Martie G. Haselton, a UCLA researcher and author of two new studies on the subject. "They just know that suddenly one day they're attracted to their hunky neighbor or handsome co-worker."

Men, meanwhile, seem to be aware on some level of this possibility and appear to step up mate-guarding strategies when their wives or girlfriends ovulate, even when neither is keeping track of the woman's cycle, the research shows.

"It's not just that men are more jealous and possessive when their partners ovulate, but they're also more attentive to their partners and more giving to their needs," said collaborator Steven W. Gangestad, a University of New Mexico psychologist.

"Although men are probably not aware of it, they behave as though they're genuinely concerned about being cuckolded," Gangestad continued. "It turns out that there's some basis for the fear."

The findings advance recent research by Gangestad that has suggested a proclivity toward infidelity during ovulation. The new research, generated in collaboration with UCLA's Center on Behavior, Evolution, and Culture, pinpoints women who may be at greatest risk. The findings also suggest an evolutionary explanation for the so-called "adaptation."

For the first study, which will be reported in the Jan. 4 scholarly journal "Hormones and Behavior," Haselton, an assistant professor of communication studies and psychology at UCLA, and Gangestad recruited 38 female coeds from a large U.S. university. (The researchers are declining to name the institution so that the subjects don't find themselves in the position of being grilled by mates who learn of the sensitive findings.)

The subjects were asked to reveal information from which their date of ovulation could be deduced. Then they were asked to rate their partner's sexual attractiveness as measured by his desirability for a fling as well as his suitability as a long-term mate, on the theory that the first would reveal his sexual attractiveness while the second would reveal his abilities as a provider or potential provider.

Finally, the subjects submitted 35 diary-like entries, rating the strength of their attractions that day to men other than their mates and the frequency and manner with which they flirted or otherwise acted out those attractions. Also on a daily basis, the co-eds rated their own sexual attractiveness, sexual desires and sense of power within their romantic relationships.

Analyzing the diaries, Haselton and Gangestad found that ovulating had a strong effect on all women. Regardless of where they stood on other measures, the co-eds felt more desirable, attractive and powerful in their relationships during the mid-point of their cycle. But women who rated their mates as more suitable for long-term involvement than a quick fling had different behavior and desires than women who considered their mates hot.

"When women were mated to men with low sexual versus investment attractiveness, they were particularly likely to experience increased attraction to men other than their partners at mid‑cycle," Haselton said.

The mates of these women, meanwhile, appeared to appreciate unconsciously what they were up against. Their wives and girlfriends reported many more acts of mate-guarding behavior than women who considered their mates to be fling-worthy.

"What is at stake is not just the loss of face or the loss of love," Haselton said. "This is about Darwinian prosperity. Males who did not successfully guard their mates are not our ancestors."

Only one group of men diverged from this pattern: They were the ones whose wives and girlfriends were the most physically attractive.

"These guys used mate-guarding tactics all the time — whether the woman was ovulating or not," Gangestad said. "If their wives and girlfriends were correct, these guys behave as though they expected the women to be snatched from them at any point."

For the second study, Haselton and Elizabeth Pillsworth, a UCLA graduate student, recruited 43 normally ovulating women, who similarly rated their partner's sexual attractiveness. They also reported their own desires and their partner's mate retention behaviors at high and low fertility. But these subjects did so on just two occasions — once on a day near ovulation and once in the non-fertile days following ovulation — not on a daily basis. And the subject's cycle was not a matter of deduction. Lab tests for a hormone surge that marks ovulation (luteinizing hormone) confirmed that women were indeed fertile in one of the reporting sessions. The researchers then looked for patterns in the women's responses.

The findings, which are forthcoming in "Evolution and Human Behavior," the flagship journal of the field of evolutionary psychology, confirm the first study: Near ovulation, these women also were more likely to fantasize about men other than their mates, but only when they didn't consider their mates to be particularly sexy.

Together the studies give a sense of the extent to which human evolutionary history may still play a role in the present.

"Since our female ancestors couldn't peer into a potential partner's genetic makeup, they had to base their decisions on physical manifestations of the presence of good genes and the absence of genetic mutations, which include masculine features such as a deep voice, muscular body, dominant behavior and sexy looks," Haselton said. "So we still feel drawn to these visible markers for what at least in the past proved to be indicators of good genes. Ancestral women who were attracted to these features produced offspring who were more successful in attracting mates and producing progeny. The legacy of the past is desire in the present."

But as any mother with mounting bills and laundry can attest, women do not look only for good genes in a potential partner.

"In the reproductive arena, women want men to contribute both quality care and good genes," Haselton said. "The problem is that there aren't many potential mates who are high in both. So many women are forced to make trade-offs."

Numerous studies have found a pronounced preference on the part of women for a kind and resourceful investing partner. Temptation appears to raise its head when the mate is not attractive as well.

The findings contradict long-held views in biological evolution about the types of mates that women find attractive and how those attractions differ from the ones experienced by men.

"According to the traditional evolution story, men value attractiveness in a mate, whereas women value resources and status," Haselton said. "But the story appears to be much more nuanced. Physical attractiveness is very important to women, but to see this you have to catch women during the narrow window of fertility within the cycle."

These phenomena also help explain at least some of the storied tension between the sexes.

"From an evolutionary perspective, what's in the best interests of a woman and her partner can be fundamentally different," Haselton said. "It may well be in her evolutionary interest to chose a different genetic partner for her offspring. What is in the male's interest is not to let this happen, ever. It's a huge evolutionary cost because he spends time and energy ensuring the survival of genes that aren't his own. So he has an opposing strategy: He's possessive and jealous."

But in an era when birth control plays a prominent role in determining the amount of genetic material that any one couple passes on to the next generation, these adaptive traits may no longer serve their evolved function.

"The temptation to cheat on one's mate may be no different from hankering for a Krispy Kreme donut and other fatty foods," Haselton said. "It is an appetite that apparently helped ancestral humans to transmit their genes to subsequent generations, but today it may only get us into trouble."

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